Australian police have concluded an operation at the Sydney's landmark Opera House, and a separate operation in the suburb of Manly on the city's north coast.
Police had locked-down the World Heritage-listed building, suspending ferry services and clearing passersby from the site in what they said was a "precautionary measure" undertaken following "information on social media." The area is usually crowded with sightseers and commuters.
The Twitter account of the city's ferry service said one ferry service had been cancelled due to the operation.
Australia is on heightened alert for attacks by home-grown militants. Authorities say they have thwarted a number of potential attacks, although there have been several "lone wolf" assaults.